In higher organisms, the process of differentiation frequently leads to a cell type which, along with all of its descendants, produces the same group of specific proteins. It is important to determine at what level control over this process is exerted--transcriptional, translational or post-translational. With Mucor, cellular differentiation is reversible; mycelia are converted to yeast by a vegetative process and, likewise, yeast convert to mycelia by a vegetative process. I have observed that in one differentiated state, (mycelia,) either of the two isozymes of pyruvate kinase may be found depending on the presence or absence of hexose. In the alternate differentiated state, (yeast,) growth occurs only in the presence of a hexose and only one isozyme of pyruvate kinase is found. In this proposal I wish to study the control and synthesis of activity of each isozyme of pyruvate kinase and to determine the relationship of isozyme type to morphogenesis in Mucor. I also intend to purify and characterize isozymes of pyruvate kinase with the goal of understanding the relationship of the isozymes to each other. The requirement of a hexose for the development of the yeast form will be related to the synthesis of cellular proteins.